This invention relates to a monitoring unit for use in protecting against line faults in an electrical power supply system and to a method for protection.
In electrical power supply systems which supply three phase voltage to a series of supply lines, it is common to provide at a supply station what is known as a ring bus arrangement for connection of the current between the lines and the station bus.
It will be appreciated that the lines can be supply lines supplying to the bus or can be feeder lines taking current from the bus and the present arrangement is designed to provide protection for the line whether it is a supply line or a feeder line.
The ring bus arrangement provides a first bus and a second auxiliary bus each of which can provide the connection for the current. Thus each line is connected to both the first bus and the auxiliary bus and can receive current from or supply current to the ring bus assembly defined by those two separate buses.
A breaker is provided between the line and each of the buses so as to disconnect current when required for the switching scheme employed at the station or in a fault condition.
For approximately twenty years there has been provided for the above system a breaker fail scheme which is responsive to failure of one of the breakers of the ring bus to properly open and to ensure that current can no longer flow between the line and either of the buses.
The protection scheme further includes current transformers or sensors which detect current flowing between the buses and the line. These currents and a signal representative of the voltage on the line are communicated to a relay which carries out various calculations of known algorithms for detecting the presence of a fault condition and for actuating opening of the breakers when required as the fault condition is detected.
In recent years developments for the protection system have improved in that the availability of digital processing has allowed a single monitoring unit to receive information concerning the current and voltage on the line and to carry out the necessary algorithms digitally for providing the output trip signal in the event of a fault condition being detected.
One example of a device of this type is manufactured by APT Power Transmissions of Manitoba Canada which is a part of NxtPHASE Corporation of BC Canada and is sold under the trademark xe2x80x9cL-Pro Line Relayxe2x80x9d. Details of this device are known to one skilled in the art and can be determined from the device itself or from literature describing the device which is publicly available.
One example of an improvement of a device of this nature is shown in co-pending application entitled xe2x80x9cMONITORING WIDE AREA DYNAMIC SWING RECORDINGS ON AN ELECTRICAL POWER SYSTEMxe2x80x9d, filed Sep. 22, 2000, Ser. No. 09/667,582, the disclosure of which is incorporated herein by reference.
The algorithms necessary for calculating a fault condition are well known and have been long established in textbooks on the subject and have been allocated device numbers as set out in standard IEEE C37.2-1979.
One example of the prior art scheme is shown and described in relation to FIG. 1 hereinafter. However this arrangement has the disadvantage that it is necessary to provide a separate breaker fail protection scheme independent of the line protection system which has a number of significant disadvantages as set forth hereinafter.
It is one object of the present invention therefore to provide an improved monitoring apparatus for use in protecting against line faults in an electrical power supply system and to a method for protection.
According to one aspect of the invention there is provided an apparatus for monitoring current flow in a power supply line of a three phase electrical power system and for controlling circuit breakers to provided protection for the system, the system comprising:
a ring bus assembly including a first bus and an auxiliary bus;
a plurality of three phase power supply lines each connected to the first bus and to the auxiliary bus for communicate power current between the respective line and the ring bus assembly;
a plurality of first circuit breakers each connected between a respective one of the lines and the first bus for disconnecting flow of current therebetween;
a plurality of second circuit breakers each connected between a respective one of the lines and the auxiliary bus for disconnecting flow of current therebetween;
a first plurality of current sensors each responsive to the three phase current flow between the first bus and a respective one of the lines to provide three outputs indicative of the A-phase, B-phase and C-phase respectively of the three phase current flow;
a second plurality of current sensors each responsive to the three phase current flow between the auxiliary bus and a respective one of the lines to provide three outputs indicative of the A-phase, B-phase and C-phase respectively of the three phase current flow;
and a plurality of voltage sensors each responsive to the voltage on a respective one of the lines to provide a voltage output indicative thereof;
the apparatus comprising a monitoring and control unit associated with a respective one of the lines having:
six current input terminals each for receiving a respective one of the three outputs of the first current sensor and the three outputs of the second current sensor associated with said respective one of the lines;
a voltage input terminal for receiving the voltage output of the voltage sensor associated with said respective one of the lines;
detector means in the unit responsive to the outputs from the terminals for generating data indicative of the instantaneous values of the voltage and of the six currents defined by the A-phase, B-phase and C-phase currents of the first sensor and the A-phase, B-phase and C-phase currents of the second sensor;
summation means in the unit for summing the two A-phases to define an A-phase summed value, the two B- phases to define an B-phase summed value and the two C-phases to define an C-phase summed value;
calculation means in the unit for effecting pre-determined algorithmic calculations on the voltage, the A-phase summed value, on the B-phase summed value and on the C-phase summed value to determine a fault condition;
a trip output generator for generating a trip output in response to said fault condition for tripping both the circuit breakers associated with said respective line;
recording means for recording data relating to said six current values in the event of a fault condition;
said calculation means being arranged, in the event of a trip output, to detect current values defined by the A-phase, B-phase and C-phase currents of the first sensor which are greater than zero and to provide a first breaker fail output for tripping all the first circuit breakers.
said calculation means being arranged, in the event of a trip output, to detect current values defined by the A-phase, B-phase and C-phase currents of the second sensor which are greater than zero and to provide a second breaker fail output for tripping all the second circuit breakers.
According to a second aspect of the invention there is provided a method comprising:
providing a ring bus assembly including a first bus and an auxiliary bus;
providing a plurality of three phase power supply lines;
connecting each three phase power supply line to the first bus and to the auxiliary bus for communicating power current between the respective line and the ring bus assembly;
locating between each one of the lines and the first bus a respective one of a plurality of first circuit breakers for disconnecting flow of current therebetween;
locating between each one of the lines and the auxiliary bus a respective one of a plurality of second circuit breakers for disconnecting flow of current therebetween;
providing a first plurality of current sensors each responsive to the three phase current flow between the first bus and a respective one of the lines to provide three outputs indicative of the A-phase, B-phase and C-phase respectively of the three phase current flow;
providing a second plurality of current sensors each responsive to the three phase current flow between the auxiliary bus and a respective one of the lines to provide three outputs indicative of the A-phase, B-phase and C-phase respectively of the three phase current flow;
providing a plurality of voltage sensors each responsive to the voltage on a respective one of the lines to provide a voltage output indicative thereof;
providing a plurality of monitoring and control units each associated with a respective one of the lines;
providing on each unit six current input terminals and connecting thereto a respective one of the three outputs of the first current sensor and the three outputs of the second current sensor associated with said respective one of the lines;
providing on each unit a voltage input terminal and connecting thereto the voltage output of the voltage sensor associated with said respective one of the lines;
in response to the outputs from the terminals, generating data indicative of the instantaneous values of the voltage and of the six currents defined by the A-phase, B-phase and C-phase currents of the first sensor and the A-phase, B-phase and C-phase currents of the second sensor;
in each unit effecting summing of the two A-phases to define an A-phase summed value, of the two B-phases to define an B-phase summed value and of the two C-phases to define an C-phase summed value;
in each unit effecting pre-determined algorithmic calculations on the voltage, the A-phase summed value, on the B-phase summed value and on the C-phase summed value to determine a fault condition;
in each unit generating a trip output in response to said fault condition for tripping both the circuit breakers associated with said respective line;
in the event of a fault condition, recording data relating to said six current values;
in the event of a trip output, detecting current values defined by the A-phase, B-phase and C-phase currents of the first sensor which are greater than zero in response thereto generating a first breaker fail output for tripping all the first circuit breakers;
and in the event of a trip output, detecting current values defined by the A-phase, B-phase and C-phase currents of the second sensor which are greater than zero in response thereto generating a second breaker fail output for tripping all the second circuit breakers.
Preferably the first breaker fail outputs are communicated directly to the first breakers and the second breaker fail outputs are communicated directly to the second breakers.
Preferably the currents are calculated in the unit to define a digital value and summed digitally.